SUMMARY: The stability of penicillinase messenger RNA has been assessed from the kinetics of enzyme production after inhibition of RNA synthesis with actinomycin or rifampicin. An average messenger half-life of 4.7 min. is indicated, both for induced Bacillus licheniformis 749, and the constitutive mutant 749/c. Although this value is twice that estimated for average total m-RNA by similar techniques, there is no indication of a ‘long-lived messenger’ as previously implicated in other reports. An attempt was made to estimate the content of penicillinase messenger from data on the level of penicillinase made per bacterium, the polysome size, the messenger half-life and the total level of unstable RNA per bacterium. It is suggested that at the maximum induced rate of enzyme formation there are approximately 110 penicillinase messengers per bacterium, and in the constitutive about twice this number.
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